CDMA or spread spectrum communications have been in existence since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications. Some examples include digital cellular radio, land mobile radio, satellite systems, and indoor and outdoor personal communication networks referred to herein collectively as cellular systems.
Currently, channel access in cellular systems is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse.
In TDMA systems, a channel consists of a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
With FDMA or TDMA systems or hybrid FDMA/TDMA systems, the goal is to insure that two potentially interfering signals do not occupy the same frequency at the same time. In contrast, Code Division Multiple Access (CDMA) allows signals to overlap in both time and frequency. Thus, CDMA signals share the same frequency spectrum in present day systems. In the frequency or the time domain, the multiple access signals appear to be on top of each other.
There are a number of advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be several times that of existing analog technology as a result of the properties of a wide band CDMA system, such as improved interference diversity, voice activity gating, and reuse of the same spectrum in interference diversity.
In principle, in a CDMA system the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or -1. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique spreading code (signature sequence). The ratio between the signature sequence bit rate and the information bit rate is called the spreading ratio.
A plurality of coded information signals modulate a radio frequency carrier, for example by quadrature phase shift keying (QPSK), and are jointly received as a composite signal at a receiver. Each of the coded signals overlaps all of the other coded signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique codes, and the corresponding information signal can be isolated and decoded.
One CDMA technique, here called "traditional CDMA with direct spreading", uses a signature sequence to represent one bit of information. Receiving the transmitted sequence or its complement (the transmitted binary sequence values) indicates whether the information bit is a "0" or "1". The signature sequence usually comprises N bits, and each bit is called a "chip". The entire N-chip sequence, or its complement, is referred to as a transmitted symbol. The receiver correlates the received signal with the known signature sequence of its own signature sequence generator to produce a normalized value ranging from -1 to +1. When a large positive correlation results, a "0" is detected; when a large negative correlation results, a "1" is detected.
Another CDMA technique, here called "CDMA with direct spreading" allows each transmitted sequence to represent more than one bit of information. A set of code words, typically orthogonal code words or bi-orthogonal code words, is used to code a group of information bits into a much longer code sequence or code symbol. A signature sequence or scramble mask is modulo-2 added to the binary code sequence before transmission. At the receiver, the known scramble mask is used to descramble the received signal, which is then correlated to all possible code words. The code word with the largest correlation value indicates which code word was most likely sent, indicating which information bits were most likely sent. One common orthogonal code is the Walsh-Hadamard (WH) code.
In CDMA, also referred to as direct sequence CDMA (DS-CDMA) to distinguish it from frequency hopping CDMA, the "information bits" referred to above can also be coded bits, where the code used is a block or convolutional code. One or more information bits can form a data symbol. Also, the signature sequence or scramble mask can be much longer than a single code sequence, in which case a sub-sequence of the signature sequence or scramble mask is added to the code sequence.
For future cellular systems, the use of hierarchical cell structures will prove valuable in even further increasing system capacity. In this cell structure, part of the wave-band of a larger cell or macro cell is devoted to smaller cells or micro cells existing within the macro cell. For instance, micro cell base stations can be placed at a lamp post level along urban streets to handle the increased traffic level in congested areas. Each micro cell might cover several blocks of a street or a tunnel, for instance. Even in CDMA systems, the different types of cells (macro and micro) would or will operate at different frequencies so as to increase the capacity of the overall system. See, H. Eriksson et at., "Multiple Access Options For Cellular Based Personal Comm.," Proc. 43rd Vehic, Tech. Soc. Conf., Secaucus, 1993. Reliable handover procedures must be supported between the different cell types, and thus between different frequencies.
In a cellular communication system such as disclosed in U.S. Pat. No. 5,101,501 to Gilhousen et at. (herein incorporated by reference), reliable handover between base stations is viable if the carrier frequency is not changed. The procedure used is called soft handover through macro-diversity, whereby the mobile station is connected to more than one base station simultaneously.
In this conventional CDMA cellular telephone system, each cell has several modulator-demodulator units or spread spectrum mediums. Each modem consists of a digital spread spectrum transmit modulator, at least one digital spread spectrum data receiver and a searcher receiver. Each modem at the base station can be assigned to a mobile station as needed to facilitate communications with the assigned mobile station. In many instances many modems are available for use while other ones may be active in communicating with respective mobile stations. In the Gilhousen system a handoff scheme is employed for a CDMA cellular telephone system in which a new base station modem is assigned to a mobile station while the old base station continues to service the call. When the mobile station is located in the transition region between the two base stations, it communicates with both base stations.
When mobile station communications are established with the new base station, e.g., the mobile station has good communications with the new cell, the old base station discontinues servicing the call. This soft handoff is in essence a make-before-break switching function. The mobile station determines the best new base station to which communications are to be transferred to from an old base station. Although it is preferred that the mobile station initiate the handoff request and determine the new base station, handoff process decisions may be made as in conventional cellular telephone systems wherein the base station determines when a handoff may be appropriate and, via the system controller, request neighboring cells to search for the mobile station signal. The base station receiving the strongest signal as determined by the system controller then accepts the handoff.
In the CDMA cellular telephone system, each base station normally transmits a "pilot carrier" signal. This pilot signal is used by the mobile stations to obtain initial system synchronization and to provide robust time, frequency and phase tracking of the base station transmitted signals.
In conventional DS-CDMA systems the mobile station is continuously occupied with receiving information from the network. In fact, DS-CDMA normally uses continuous reception and transmission in both link directions. Unlike TDMA, there are no idle time slots available to switch to other carrier frequencies, creating two problems related to handover between frequencies.
The first is reliable handover evaluation, i.e., the procedure of deciding whether to handover to a given base station on a given frequency is appropriate at a particular instant. Since the mobile station can not provide any inter-frequency measurements to a handover evaluation algorithm in the network or the mobile station, the handover decision will be made without full knowledge of the situation of the mobile station, and therefore can be unreliable.
The second problem is the handover execution. When a decision has been made to handover a call to another base station on another carrier frequency, the mobile station must drop the existing link (or links when the system is operating with macro-diversity), switch to the new carrier frequency, and initiate a new link. When the mobile station and the new base station are establishing synchronization, information will be lost and the call quality will degrade.
These problems could be solved by implementing two receivers in the mobile station, but this would involve an undesirable increase the amount of RF hardware needed.